There has been known an SCR (Selective Catalytic Reduction) system equipped with a selective catalytic reduction catalyst provided in an exhaust passage of an internal combustion engine and a reducing agent addition valve provided in the exhaust passage upstream of the selective catalytic reduction catalyst, in which a compound or composition derived from ammonia is added through the aforementioned reducing agent addition valve.
With increasingly strict exhaust emission control, there arises a need for adding an oxidation catalyst and/or particulate filter to SCR systems. Thus, systems equipped with a fuel addition valve, an oxidation catalyst, a particulate filter, a urea addition valve, a selective catalytic reduction catalyst, and an ammonia oxidation catalyst arranged in an exhaust passage in order from upstream have been developed (see, for example, Patent Document 1).
Patent Document 2 describes a system in which a urea addition valve, an oxidation catalyst, a particulate filter, and a selective catalytic reduction catalyst are arranged in an exhaust passage in order from upstream. Patent Document 3 describes a system in which a particulate filter and a selective catalytic reduction catalyst are arranged in an exhaust passage in order from upstream, and urea and fuel are added at a location between the particulate filter and the selective catalytic reduction catalyst. Patent Document 4 describes an exhaust gas purification catalyst having an oxidizing ability and a selective reduction ability.